1. Field of the Invention
This invention relates to a process for optical transmission of signals by self-heterodyning and a transmission system with a switching matrix using such a process.
2. Discussion of Background
A system for transmission by self-heterodyning consists, as can be seen in FIG. 1, of an emission means, a transmission means, and a reception means.
The emission means include a monofrequency laser 1 controlled by feed circuits 2 followed by a collimator 3 to collimate the beam emitted by the monofrequency laser. These means further comprise an acoustooptic modulator 4 energized by a frequency generator, to separate the incident beam in two separate beam. The beam is divided into a nondeflected beam following path B and having the same frequency as the incident beam and into a deflected beam following path A and of a frequency equal to the frequency of the incident beam plus or minus the excitation frequency of the acoustooptic modulator. The + or - sign depends on the relative movement of the optical and acoustic waves.
An electrooptic modulator 7 makes it possible to amplitude modulate the deflected optical signal.
At the output of the modulator, the optical signal is delayed by a polarization-preserving fiber 8. This delay T is greater than or equal to 1/.DELTA.F, .DELTA.F being the spectral width of the emission of laser 1. The signal then passes into a polarization controller 9.
This polarization controller 9 makes a final adjustment of the polarization direction of the beam of path A on that of path B.
The output of polarization controller 9 is connected to an arm of an optical mixer 11.
The nondeflected beam is focused by a lens 16 in the second arm of mixer 11.
The two waves of paths A and B, at the output of mixer 11, have the same polarization direction.
These signals are carried by the transmission means which consist of a monomode transmission fiber 12.
The reception means consist of a quadratic photodetector 13 followed by an amplifying and demodulating circuit 14.
The mixing of the two waves is performed at the level of the quadratic detector which detects the beat signal at intermediate frequency f1 which is the same frequency used to control acoustooptic modulator 4.
This signal with intermediate frequency f1 contains the data provided by amplitude modulator 7. This data is extracted from the signal with intermediate frequency f1 by amplifying and demodulating circuits 14.
Since the frequencies transmitted on path A containing the data-carrying signal and on path B containing the signal from the local oscillator are very close, the modifications of the polarization directions produced by the transmission fiber are identical for the two beams which are found with polarization directions at the level of photodetector 13.
When it is desired to increase the power of the local oscillator, a traveling wave optical amplifier 15 is placed on path B in the emission means.
The means of reception by "standard" heterodyne detection therefore comprise a local oscillator delivering a wave at such a frequency that the mixing of the data-carrying incident signal and the signal from the local oscillator makes it possible to obtain a wave at the intermediate frequency. To obtain a perfectly stabilized wave, it is necessary to use an automatic frequency control to act on the optical frequency of the local oscillator to stabilize it.
On the other hand, it is necessary that the polarization direction of the two optical waves be the same for the two incident and local waves. Since the optical fiber does not maintain this direction, it is usual to use, at reception, a circuit to correct and control the incident polarization. It can involve either a polarization controller or a double detection with polarization diversity.
This invention has as its object to eliminate these drawbacks. It further makes it possible to simplify the reception equipment. This advantage is significant in transmission systems with a switching matrix, such systems being used for local networks, because it leads to a reduction in the amount of subscriber equipment required.